1. Field of the Invention
The present invention relates to an integrated optical device which performs optical data processing in an integrated arrangement using an optical waveguide, and, more particularly, to an integrated optical device which is suitable for use in a wavelength wave divider and an optical spectrum analyzer.
2. Description of the Prior Art
In recent years, research has been actively carried out on integrated optical devices which optically perform modulation, deflection, switching, arithmetic processing, and the like in a compact arrangement using an optical waveguide. In such devices, there is an interaction between a light beam guided through a slab optical waveguide and a surface acoustic wave. In such cases, the guided light beam is preferably a parallel light beam and the slab optical waveguide functions to confine the light solely in a direction perpendicular to the surface of a substrate which forms the waveguide. Such a waveguide is also called a two-dimensional optical waveguide or a planar optical waveguide.
To obtain a parallel light beam in a conventional integrated optical device, an arrangement such as that shown in FIG. 1 has been employed. In the Figure, reference numeral 2 denotes a substrate, while reference numeral 4 denotes a slab optical waveguide formed on the surface thereof. An optical waveguide lens 36 is formed in the slab optical waveguide 4 and may, for example, be a geodesic lens, a Luneburg lens, or a Fresnel lens. A semiconductor laser 8 is connected to an end surface of the substrate 2. Since the semiconductor laser 8 is disposed at the focal point of the optical waveguide lens 36, a divergent light beam 38 introduced into the slab optical waveguide 4 from the semiconductor laser 8 passes through the optical waveguide lens 36 and is thereby collimated to become a parallel guided light beam 40.
In addition, an integrated optical device such as described above has been used as a wavelength wave divider and as an optical spectrum analyzer. Examples of such uses will now be described.
FIG. 2 is a schematic perspective view illustrating an example of a conventional wavelength wave divider. In the Figure, a collimating optical waveguide lens 60, wave-dividing gratings 62a, 62b, converging optical waveguide lenses 64a, 64b, and photodetectors 66a, 66b are disposed in the slab optical waveguide 4 formed on the substrate 2. In addition, the wave-dividing gratings 62a, 62b have different wavelength selection characteristics. An optical fiber 68 for introducing input light is coupled to an end surface of the substrate 2. Since the coupling position is arranged so as to become the focal point of the collimating optical waveguide lens 60, a divergent light beam 70, generated when multiple wavelength optical signals from the optical fiber 68 are introduced into the slab optical waveguide 4, is collimated into a parallel guided beam 72 by passing through the optical waveguide lens 60. Subsequently, the guided beam 72 having a plurality of wavelength components is diffracted by means of the gratings 62a, 62b to generate diffracted guided beams 74a, 74b. The diffracted guided beams 74a, 74b have wavelengths which substantially satisfy the Bragg condition. In other words, assuming that the pitches of the gratings 62a, 62b are .LAMBDA.a, .LAMBDA.b, respectively, selected wavelengths .lambda.a, .lambda.b can be given by the following formula: EQU .lambda.i=2Ni.lambda.i sin.theta.i
where i is a or b; Ni is a standardization propagation constant (an effective refractive index); and .theta.i is a Bragg angle (2.theta.i is a deflection angle).
The light beams 74a, 74b with the selected and spatially separated wavelengths .lambda.a, .lambda.b are converged, respectively by condenser lenses 64a, 64b and are made incident upon photodetectors 66a, 66b so as to be detected.
FIG. 3 is a schematic perspective view illustrating an example of the arrangement of a conventional integrated optical spectrum analyzer (IOSA).
In the Figure, a collimating optical waveguide lens 76, an optical waveguide lens 78 for Fourier transformation, a photodetector array (e.g., CCD) 80, and a comb electrode 82 for exciting surface acoustic waves are disposed in the slab optical waveguide 4 formed on the surface of the substrate 2. A semiconductor laser 84 is coupled with an end surface of the substrate 2. The semiconductor laser 84 is disposed at the focal point of the collimating optical waveguide lens 76, while the photodetector array 80 is disposed at the focal point of the optical waveguide lens 78 for Fourier transformation. A divergent light beam 86, which is introduced from the semiconductor laser 84 into the slab optical waveguide 4, is collimated into a parallel guided beam 88 by passing through the optical waveguide lens 76. A surface acoustic wave 90 is excited by applying a radio frequency (RF) signal to the comb electrode 82, and the collimated parallel beam 88 is subjected to Bragg diffraction by means of the surface acoustic wave 90. An angle of diffraction is determined in response to the wavelength of the surface acoustic wave 90, i.e., the frequency of the applied RF signal. Thus, since the image-forming position changes in response to the angle of diffraction by causing the diffracted beam 92 to form an image at the photodetector array 80 by means of the Fourier transform lens 78, the spectrum of the diffracted beam, i.e., the spectrum of the RF signal, can be obtained on a real-time basis.
However, the conventional integrated optical devices described above have suffered from the following drawbacks.
First, since an optical waveguide lens is employed on the input and output sides, respectively, there is a limit to the compactness of the device. For instance, when an input light beam from the semiconductor laser 8 is made into a parallel light beam, as shown in FIG. 1, since the angle of divergence of the divergent light beam from the semiconductor laser 8 is fixed, it becomes necessary to provide a large distance between the semiconductor laser 8 and the optical waveguide lens 36 in order to obtain a wide parallel beam (namely, it is necessary to use a lens having a large focal length). In addition, in an optical spectrum analyzer such as shown in FIG. 3, a large-aperture lens is required to effect a spectrum analysis with high resolving power, so that the device is inevitably large in size.
Second, in order to improve the accuracy of the degree of parallelism, etc., of the collimated light beam, it is necessary to place the focal point of the optical waveguide lens at a desired photocoupling position, which requires high-level alignment techniques, with the result that fabrication is difficult and the yield is low.
Third, although various types of waveguide lenses have been proposed, none of them has a large degree of freedom in design in terms of focal length, aperture, and the like, with the result that restrictions are imposed on the specifications of the device.
Furthermore, in the case of the spectrum analyzer shown in FIG. 3, there also has been a problem in that, in order to obtain high resolving power, it is necessary to reduce the spatial resolution, i.e., the bit size, of the photodetector array 80, thereby making fabrication difficult.
A technique for obtaining a wide parallel light beam in a compact arrangement by monolithically forming a semiconductor laser and a grating coupler on an optical waveguide has been proposed in H. M. Stoll "High-Brightness Lasers Using Integrated Optics," SPIE 139, pp. 113-116 (1978). In the grating coupler described in that paper, however, a grating structure is formed on a slab optical waveguide. Such an arrangement has a drawback in that, since the light beam from the semiconductor laser is a divergent light beam, the diffraction efficiency is reduced, and it is therefore difficult to input a beam at a sufficient level of efficiency.